Photovoltaic devices, or solar cells, are devices which convert light, especially sunlight, into direct current (DC) electrical power. For low-cost mass production, thin film solar cells are of interest since they allow using glass, glass ceramics or other rigid or flexible substrates instead of crystalline or polycrystalline silicon as a base material, or substrate, respectively. Various solar cell technologies are commercially available today. The possibility to process such cells at low temperatures and in a large scale is a major advantage of this technology.
The solar cell structure, i. e. the layer sequence, is responsible for or capable of the photovoltaic effect. The layers may be coated, or deposited, respectively, as thin layers by means of known vacuum deposition techniques such as PVD, CVD, PECVD, APCVD, all of which may be used in semiconductor technology.
Conventional thin-film solar cells usually comprise a transparent electrode layer, also called front electrode, deposited on a substrate. On top of this first electrode layer, a photoelectric conversion semiconductor layer formed of a thin amorphous and/or microcrystalline silicon film and a back electrode layer are usually deposited. Said back electrode may again comprise a transparent conductive layer as well as a reflector layer, a conductive and reflective metal layer or a technical equivalent thereof.
In detail, prior Art FIG. 1 shows a basic, simple photovoltaic cell 10 comprising a transparent substrate 12, with a layer of a transparent conductive oxide (TCO) 14 deposited thereon. This layer is also called front contact and acts as first electrode for the photovoltaic element. The combination of substrate 12 and front contact 14 is also known as superstrate. The next layer 16 acts as the active photovoltaic layer and exhibits three “sub-layers” forming a p-i-n junction. Said layer 16 comprises hydrogenated microcrystalline, nanocrystalline or amorphous silicon or a combination thereof. Sub-layer 18, which is arranged adjacent to the TCO front contact 14, is positively doped, the adjacent sub-layer 20 is intrinsic, and the final sub-layer 22 is negatively doped. In an alternative embodiment, the layer sequence p-i-n as described can be inverted to n-i-p. In this case, layer 16 is identified as n-layer, layer 20 again as intrinsic, layer 22 as p-layer.
Finally, the cell includes a rear contact layer 24, which also is called back contact. Additionally, a reflective layer 26 is provided. Alternatively a metallic back contact may be realized, which can combine the physical properties of back reflector 26 and back contact 24. For illustrative purposes, arrows indicate impinging light.
It is generally understood that when light, for example, solar radiation, impinges on a photoelectric device, e.g. a solar cell, electron-hole pairs are generated in the i-layer. The holes from the generated pair are directed towards the p-region and the electrons towards the n-region. The contacts are generally directly or indirectly in contact with the p- and n-regions. Current will flow through an external circuit connecting these contacts as long as light continues to generate electron-hole pairs.
Transparent conducting (TC) layers, to be used as layers 14 and/or 24 as described above, are thin film materials that combine optical transparency in the visible spectral range with electrical conductivity suitable for optoelectronic applications.
Numerous applications such as defrosting windows or transparent electrodes for flat panel displays, and solar cells use transparent conducting layers. It is usually mandatory in these applications to maximize both the conductance and the transmittance of transparent conducting layers.
Various processes may produce transparent conducting layers. The most commonly used techniques are: sputtering (PVD), chemical vapour deposition (CVD), pulsed laser deposition, spray pyrolysis, and wet dip deposition. These processes employ diverse materials as substrates, for instance glass, plastic foils or alike. These materials may be flexible, or rigid.
In order to improve the electrical conversion efficiency of a photovoltaic device, as much as possible of the impinging light shall be able to be absorbed within the active silicon layers.
Transparent conducting material properties, or high conductance combined with high transmittance, respectively, are generally difficult to achieve simultaneously because optical transparency in the visible range often may require materials with band gaps larger than 3.3 eV and such large band gap materials render carrier doping and thus high conductivity difficult to realize.
In addition to this fundamental limitation, fabrication processes usually induce defects in the layers that limit the transparency and the conductivity. Such defects are for instance: porosity, grain boundaries and impurity contamination.
Known from U.S. Pat. No. 6,420,644 B1 is a solar battery having a board with a surface and a plurality of spherical segments projecting from the board surface. To define the plurality of spherical segments, the board surface may be embossed. On top of the board, a first electrode made from chrome may be provided.
Known from U.S. 2005/0022860 A1 is a thin-film photovoltaic module. To improve the utilization of such a module, a plurality of substantially hemispheric protrusions is provided on a substrate. In case the substrate is a metallic substrate, the protrusions are formed by embossing a metal substrate. In case a plastic substrate is provided, the protrusions may be formed by injection molding.